Vasovagal syncope (VVS, simple faint) is the most common cause of transient loss of consciousness and is the acute episodic form of orthostatic intolerance (OI). Postural tachycardia syndrome (POTS) is the common chronic form of OI. Both are defined by debilitating symptoms and signs while upright relieved by recumbency. Pathophysiological mechanisms have remained elusive although our past work shows that excessive upright central hypovolemia results from splanchnic blood pooling due to defective splanchnic arterial and venous constriction. Preliminary data support the hypothesis that production of nitric oxide (NO) is enhanced in these patients resulting in reduced sympathetic noradrenergic neurotransmission at pre-junctional and post-junctional sites. Our approach is two-fold: 1) We will use intradermal microdialysis and laser Doppler flowmetry (LDF) to delineate the microvascular mechanisms of NO modulation of noradrenergic neurotransmission free of confounding systemic reflex changes. 2) We will systemically apply this mechanism to a model of orthostatic stress, lower body negative pressure (LBNP), while measuring cardiac output by inert gas rebreathing, regional blood volume, and regional blood flow using plethysmographic techniques focusing on splanchnic changes, and muscle sympathetic nerve activity by peroneal microneurography. 30 each of VVS, POTS and control subjects ages 14-29 years will be recruited. We will use chemiluminescence to measure NO in microdialysate to test whether it is increased in OI. Pre-junctional neurotransmission will be assessed by step wise tyramine doses using HPLC analysis of microdialysate norepinephrine (NE). Post-junctional neurotransmission will be assessed by LDF responses to step-wise increases of NE after endogenous blockade with bretylium. Basal and local heat stimulated NO will be assessed. We will determine whether inhibition of neurotransmission is NO-specific by repeating studies after NOS inhibition with nitro-L-arginine (NLA) and after repletion of NO during perfusion with NLA+sodium nitroprusside (SNP). We will obtain skin biopsies for NOS isoform protein and expression. We will determine if systemic NO inhibits pre and post synaptic adrenergic activity in OI, altering the response to LBNP. Presynaptic neurotransmission will be assessed by step-wise increasing LBNP and measuring plasma NE response. Post-synaptic neurotransmission will be assessed by the systemic response to stepwise increasing phenylephrine doses. To confirm NO-specificity, studies will be repeated after NOS inhibition with L-NG-monomethyl Arginine(L-NMMA), after repletion of NO during perfusion with L-NMMA +SNP, and after low dose phenylephrine as a control for baseline shifts of MSNA and vasoconstriction. L-NMMA enters the CNS slowly over many hours. To ascertain its central effects we will measure MSNA at the end of experiments. Studies will establish the molecular mechanism for common and debilitating forms of OI in the cutaneous human surrogate model, and apply this knowledge to the integrative physiology of OI. This research will lead to targeted and effective medical therapy for important groups of OI patients.